Full Idea
It is said that a good explanation ought to show that the explanandum event had to happen, given the laws and circumstances.
Clarification
The 'explanandum' is the thing to be explained
Gist of Idea
A good explanation is supposed to show that the event had to happen
Source
David Lewis (Causal Explanation [1986], V)
Book Reference
Lewis,David: 'Philosophical Papers Vol.2' [OUP 1986], p.228
A Reaction
I cautiously go along with this view. Given that there are necessities in nature (a long story), we should aim to reveal them. There is no higher aspiration open to us than successful explanation. Lewis says good explanations can reveal falsehoods.